With the technology development and the growing demands for mobile devices, the demand for secondary batteries as an energy source is dramatically increasing, and among the secondary batteries, a lithium secondary battery having high energy density and operating potential, a long cycle life, and a low self-discharging rate has been commercialized and is being widely used.
Also, recently, with the growing interest in environmental issues, many studies are being conducted on electric vehicles (EVs) and hybrid electric vehicles (HEVs) other than vehicles running on fossil fuels, such as gasoline vehicles and diesel vehicles, attributable to air pollution.
Electric vehicles (EVs) and hybrid electric vehicles (HEVs) use Ni-MH secondary batteries or lithium secondary batteries having a high energy density, a high discharge voltage and output stability as a power source, and when a lithium secondary battery is used in an electric vehicle, the lithium secondary battery needs to be used for ten or more years under a severe condition along with a high energy density and capability of providing a high output in a short time, and thus is necessarily required to have much better stability and long-term life characteristics than an existing small-sized lithium secondary battery. Also, a secondary battery in use for an electric vehicle (EV) and a hybrid electric vehicle (HEV) needs to be excellent in rate characteristics and power characteristics based on an operating condition of the vehicle.
Currently, as a cathode active material of a lithium ion secondary battery, lithium-containing cobalt oxide such as LiCoO2 of a layered structure, lithium-containing nickel oxide such as LiNiO2 of a layered structure, and lithium-containing manganese oxide such as LiMn2O4 of a spinel crystal structure are being used.
LiCoO2 has excellent material properties including excellent cycle characteristics and is being widely used at present, but has low safety and because cobalt as a raw material is a finite resource, is costly and insufficient to use in large amounts as a power source in the field of industries such as electric vehicles. According to characteristics of a manufacturing method, applying LiNiO2 to a mass production process at a reasonable cost is impractical.
In contrast, lithium manganese oxide such as LiMnO2 and LiMn2O4 has an advantage of using manganese noted for an abundant and eco-friendly resource as a raw material, and thus is attracting much attention as an alternative cathode active material to LiCoO2. However, these exemplary lithium manganese oxides have also a shortcoming of poor cycle characteristics. LiMnO2 has a drawback of low initial capacity and in that several tens of charging and discharging cycles are needed until it reaches a predetermined capacity. Also, LiMn2O4 experiences a serious capacity reduction during cycles, and particularly, has a disadvantage of drastic degradation in cycle characteristics due to decomposition of an electrolyte solution and manganese release at high temperature higher than or equal to 50° C.
In this context, lithium nickel-manganese-cobalt-based composite oxide of a layered structure is proposed as a good active material having excellent battery performance balance while overcoming or minimizing the problems of each cathode active material. However, lithium nickel-manganese-cobalt-based composite oxide of a layered structure needs improvements of, in particular, rate characteristics for a wide range of applications.